Spider silk proteins and methods for producing spider silk proteins

ABSTRACT

The invention provides an isolated major ampullate spidroin protein, which consists of from 150 to 420 amino acid residues and is defined by the formula REP-CT. REP is a repetitive, N-terminally derived protein fragment having from 80 to 300 amino acid residues. CT is a C-terminally derived protein fragment having from 70 to 120 amino acid residues. The invention further provides an isolated fusion protein consisting of a first protein fragment, which is a major ampullate spidroin protein, and a second protein fragment comprising a fusion partner and a cleavage agent recognition site. The first protein fragment is coupled via said cleavage agent recognition site to the fusion partner. The invention also provides a method of producing a major ampullate spidroin protein and polymers thereof.

This application is a Continuation of application Ser. No. 12/087,289 filed Jun. 30, 2008, now U.S. Pat. No. 8,173,772 which is the National phase of PCT International Application No. PCT/SE2006/001505 filed on Dec. 28, 2006. This application also claims priority to Patent Application No. 0502932-7 filed in Sweden on Dec. 30, 2005. All of the above applications are hereby expressly incorporated by reference into the present application.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the field of recombinant production of proteins. More specifically, the present invention is concerned with recombinant production of spider silk proteins. The present invention provides novel isolated major ampullate spidroin proteins and major ampullate spidroin fusion proteins, as well as methods and polynucleic acid molecules for producing such proteins. There is also provided polymers of the major ampullate spidroin proteins and methods for producing such polymers.

BACKGROUND OF THE INVENTION

Spider silks are nature's high-performance polymers, obtaining extraordinary toughness due to a combination of strength and elasticity. Up to seven specialized glands exist in spiders, which produce a variety of silk fiber types with different mechanical properties and functions. Dragline silk, produced by the major ampullate gland, is the toughest fiber, and on a weight basis it outperforms man-made materials, such as high tensile steel and Kevlar. The properties of dragline silk are attractive in development of new materials for medical or technical purposes.

Dragline silk consists of two main polypeptides, mostly referred to as major ampullate spidroin (MaSp) 1 and 2, but to ADF-3 and ADF-4 in Araneus diadematus. These proteins have apparent molecular masses in the range of 200-720 kDa, depending on sample age and conditions of analysis, but no full-length dragline spider silk gene has yet been reported. The properties of dragline silk polypeptides are discussed in Huemmerich, D. et al. Novel assembly properties of recombinant spider dragline silk proteins. Curr. Biol. 14, 2070-2074 (2004). The known dragline silk spidroins are composed of highly iterated blocks of alternating alanine-rich segments, forming crystalline β-sheets in the fiber, and glycine-rich segments which are more flexible and mainly lack ordered structure. The C-terminal region is non-repetitive, highly conserved between species, and adopts α-helical conformation. The N-terminal region of dragline silk proteins has not been characterized until very recently, revealing an N-terminal domain that is highly conserved between different spidroins, and also between different spider species (Rising, A. et al. N-terminal nonrepetitive domain common to dragline, flagelliform, and cylindriform spider silk proteins. Biomacromolecules 7, 3120-3124 (2006)).

The mechanical properties of dragline silk varies between species; Euprosthenops sp dragline silk is stiffer, stronger (requires more force to break) and less extendible than dragline silk from e.g. Araneus diadematus or Nephila clavipes. Dragline silk from Euprosthenops sp appears to have a greater proportion of crystalline β-sheet structure than dragline silk from Araneus diadematus, most likely due to that the Euprosthenops sp MaSp has the highest polyalanine content among all species analyzed so far (Pouchkina-Stantcheva, N. N. & McQueen-Mason, S. J. Molecular studies of a novel dragline silk from a nursery web spider, Euprosthenops sp. (Pisauridae). Comp Biochem Physiol B Biochem Mol Biol 138, 371-376 (2004)).

Attempts to produce artificial spider silks have employed natural or synthetic gene fragments encoding dragline silk proteins, since no full-length gene has yet been reported. Recombinant dragline silk proteins have been expressed in various systems including bacteria, yeast, mammalian cells, plants, insect cells, transgenic silkworms and transgenic goats. See e.g. Lewis, R. V. et al. Expression and purification of a spider silk protein: a new strategy for producing repetitive proteins. Protein Expr. Purif. 7, 400-406 (1996); Fahnestock, S. R. & Irwin, S. L. Synthetic spider dragline silk proteins and their production in Escherichia coli. Appl. Microbiol. Biotechnol. 47, 23-32 (1997); Arcidiacono, S. et al. Purification and characterization of recombinant spider silk expressed in Escherichia coli. Appl. Microbiol. Biotechnol. 49, 31-38 (1998); Fahnestock, S. R. & Bedzyk, L. A. Production of synthetic spider dragline silk protein in Pichia pastoris. Appl. Microbiol. Biotechnol. 47, 33-39 (1997); and Lazaris, A. et al. Spider silk fibers spun from soluble recombinant silk produced in mammalian cells. Science 295, 472-476 (2002).

WO 2004/016651 (The University of York) discloses nucleic acid sequences coding for internal, repetitive parts of MaSp1 proteins from Euprosthenops sp. No protein is expressed.

Huemmerich, D. et al. Primary structure elements of spider dragline silks and their contribution to protein solubility. Biochemistry 43, 13604-13612 (2004) discloses a synthetic gene, “(AQ)₁₂NR3”, coding for repetitive Ala-rich and Gly/Gln-rich fragments and a non-repetitive fragment, all derived from ADF3 from Araneus. The gene is expressed into a soluble protein (59.8 kD, >528 aa), which aggregates but does not form polymers or fibers. The alanine content of the protein is 10-15%.

WO 03/057727 discloses expression of soluble recombinant silk polypeptides in mammalian cell lines and animals. One expressed silk polypeptide (ADF-3; 60 kD, 652 aa) consists of a repetitive unit and a non-repetitive hydrophilic domain. Another expressed silk polypeptide (ADF-3 His; 63 kD, 677 aa) consists of a repetitive unit, a non-repetitive hydrophilic domain, a c-myc epitope and a six-Histidine tag. The repetitive unit has a low content of Ala (10-20%). The obtained silk polypeptides exhibit poor solubility in aqueous media and/or form precipitates. Since the obtained silk polypeptides do not polymerize spontaneously, spinning is required to obtain polymers or fibers.

Several factors complicate the expression of dragline silk proteins. Due to the highly repetitive nature of the genes, and the concomitant restricted amino acid composition of the proteins, transcription and translation errors occur. Depletion of tRNA-pools in microbial expression systems, with subsequent discontinuous translation, leading to premature termination of protein synthesis might be another reason. Other reasons discussed for truncation of protein synthesis are secondary structure formation of the mRNA, and recombination of the genes. Native MaSp genes larger than 2.5 kb have been shown to be instable in bacterial hosts. Additionally, there are difficulties in maintaining the recombinant silk proteins in soluble form, since both natural-derived dragline silk fragments and designed block copolymers, especially MaSp1/ADF-4-derived proteins, easily self-assemble into amorphous aggregates, causing precipitation and loss of protein. See Huemmerich, D. et al. Primary structure elements of spider dragline silks and their contribution to protein solubility. Biochemistry 43, 13604-13612 (2004) and Lazaris, A. et al. Spider silk fibers spun from soluble recombinant silk produced in mammalian cells. Science 295, 472-476 (2002).

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a novel spider silk protein, which can provide spider silk fibers.

It is another object of the present invention to provide a water-soluble spider silk protein, which can readily be manipulated to self-polymerize into fibers at wish. This allows for unique applications, such as culturing of eukaryotic cells on the fibers. Furthermore, this property allows for all the following steps to be undertaken under physiological conditions, which decreases the risk for toxicity and protein denaturation.

It is yet another object of the present invention to provide fibers of a novel spider silk protein.

It is one object of the present invention to provide spider silk proteins in large scale, which proteins can readily be manipulated to self-polymerize into fibers at wish.

It is also an object of the invention to provide methods of producing silk proteins and fibers of spider silk proteins.

For these and other objects that will be evident from the following disclosure, the present invention provides according to one aspect an isolated major ampullate spidroin protein, wherein the protein consists of from 150 to 420 amino acid residues and is defined by the formula REP-CT, wherein REP is a protein fragment having from 80 to 300 amino acid residues, wherein said fragment is selected from the group of L(AG)_(n)L (SEQ ID NO: 17), L(AG)_(n)AL (SEQ ID NO: 18), L(GA)_(n)L (SEQ ID NO: 19), L(GA)_(n)GL (SEQ ID NO: 20), wherein n is an integer from 4 to 8;

each individual A segment is an amino acid sequence of from 8 to 18 amino acid residues, wherein from 0 to 3 of the amino acid residues are not Ala, and the remaining amino acid residues are Ala; each individual G segment is an amino acid sequence of from 12 to 30 amino acid residues, wherein at least 40% of the amino acid residues are Gly; and each individual L segment is a linker amino acid sequence of from 0 to 20 amino acid residues; and CT is a protein fragment having from 70 to 120 amino acid residues, which fragment is a C-terminal fragment derived from a major ampullate spidroin protein, or a derivative thereof.

The present invention is based on the identification of a protein motif, which is sufficient to form silk-like fibers, and the use of said motif for construction of recombinant MaSp proteins, which are possible to produce in suitable hosts, such as bacteria, preferably E. coli.

In certain embodiments according to the invention, each individual A segment has at least 80% identity to an amino acid sequence selected from the group of amino acid residues 7-19, 43-56, 71-83, 107-120, 135-147, 171-183, 198-211, 235-248, 266-279, 294-306, 330-342, 357-370, 394-406, 421-434, 458-470, 489-502, 517-529, 553-566, 581-594, 618-630, 648-661, 676-688, 712-725, 740-752, 776-789, 804-816, 840-853, 868-880, 904-917, 932-945, 969-981, 999-1013, 1028-1042 and 1060-1073 of SEQ ID NO: 3; amino acid residues 31-42, 61-75, 90-104, 122-135 and 153-171 of SEQ ID NO: 9; amino acid residues 12-25, 46-60, 75-88, 112-119, 150-158 and 173-180 of SEQ ID NO: 13; amino acid residues 31-42 of SEQ ID NO: 14; and amino acid residues 122-135 of SEQ ID NO: 15. In specific embodiments, each individual A segment is an amino acid sequence selected from this group of amino acid sequences.

In some embodiments according to the invention, each individual G segment has at least 80% identity to an amino acid sequence selected from the group of amino acid residues 20-42, 57-70, 84-106, 121-134, 148-170, 184-197, 212-234, 249-265, 280-293, 307-329, 343-356, 371-393, 407-420, 435-457, 471-488, 503-516, 530-552, 567-580, 595-617, 631-647, 662-675, 689-711, 726-739, 753-775, 790-803, 817-839, 854-867, 881-903, 918-931, 946-968, 982-998, 1014-1027, 1043-1059 and 1074-1092 of SEQ ID NO: 3; SEQ ID NO: 5; SEQ ID NO: 6; SEQ ID NO: 7; amino acid residues 11-30, 43-60, 76-89, 105-121 and 136-152 of SEQ ID NO: 9; and amino acid residues 1-11, 26-45, 61-74, 89-111, 120-149 and 159-172 of SEQ ID NO: 13. In specific embodiments, each individual G segment is identical to an amino acid sequence selected from this group of amino acid sequences.

In certain embodiments according to the invention, said CT fragment has at least 50% identity to SEQ ID NO: 8 or at least 80% identity to an amino acid sequence selected from the group consisting of SEQ ID NO: 4, amino acid residues 172-269 of SEQ ID NO: 9, amino acid residues 181-276 of SEQ ID NO: 13 and amino acid residues 172-269 of SEQ ID NO: 16 as well as any amino acid sequence of FIG. 3, in particular the MaSp1 sequences of FIG. 3. In specific embodiments, said CT fragment is an amino acid sequence selected from this group of amino acid sequences.

In certain embodiments according to the invention, the content of lipopolysaccharides (LPS) and other pyrogens in the isolated major ampullate spidroin protein is 1 endotoxin unit (EU)/mg protein or lower.

According to another aspect, the present invention provides an isolated fusion protein consisting of a first protein fragment, which is a major ampullate spidroin protein, and a second protein fragment, wherein said second protein fragment comprises a fusion partner and a cleavage agent recognition site, wherein said first protein fragment is coupled via said cleavage agent recognition site to said fusion partner.

The present invention provides an isolated fusion protein selected from the group of X-REP-CT, and REP-CT-X, wherein REP and CT are protein fragments according to the invention; and X is a protein fragment comprising a fusion partner and a cleavage agent recognition site; wherein the combined protein fragment REP-CT is coupled via said cleavage agent recognition site to said fusion partner.

In certain embodiments according to the invention, the content of LPS and other pyrogens in the isolated fusion protein is 1 EU/mg protein or lower.

According to yet another aspect, the present invention provides a method of producing a major ampullate spidroin protein according to the invention, comprising the steps of: (i) providing a solution of a fusion protein according to the invention in a liquid medium, (ii) adding to said liquid medium a suitable cleaving agent for achieving cleavage of the fusion protein at the cleavage agent recognition site, and thereby obtaining the major ampullate spidroin protein; and optionally (iii) isolating the major ampullate spidroin protein obtained in step (ii) from said liquid medium.

The present invention also provides a method of producing a polymer of a major ampullate spidroin protein according to the invention, comprising the steps of: (i) providing a solution of a fusion protein according to the invention in a liquid medium, (ii) adding to said liquid medium a suitable cleaving agent for achieving cleavage of the fusion protein at the cleavage agent recognition site, and thereby obtaining the major ampullate spidroin protein; (iii) allowing the major ampullate spidroin protein obtained in step (ii) to polymerize in the liquid medium; and optionally (iv) isolating the polymer obtained in step (iii) from said liquid medium. In a preferred method, said step (iii) further comprises providing an interface between said liquid medium and another phase selected from the group consisting of a gas phase, a liquid phase and a solid phase, wherein said polymerizing initiates at said interface or in a region surrounding said interface. In a preferred method, said liquid medium is an aqueous medium and said other phase is selected from the group consisting of air and water-immiscible organic solvents.

According to another aspect, the present invention provides an isolated polynucleic acid molecule comprising a nucleic acid sequence which encodes a major ampullate spidroin protein according to the invention, or its complementary nucleic acid sequence.

According to yet another aspect, the present invention provides an isolated polynucleic acid molecule comprising a nucleic acid sequence which encodes a fusion protein according to the invention, or its complementary nucleic acid sequence.

Another aspect of the invention resides in a method of producing a soluble fusion protein according to the invention, comprising the steps of: (i) expressing a polynucleic acid molecule encoding a soluble fusion protein according to the invention in a suitable host; and (ii) isolating the soluble fusion protein obtained in step (i). Optionally, said step (ii) of isolating the soluble fusion protein involves removal of LPS and other pyrogens.

The present invention also provides a method of producing a major ampullate spidroin protein according to the invention, comprising the steps of: (i) expressing a polynucleic acid molecule encoding a soluble fusion protein according to the invention in a suitable host; (ii) isolating the soluble fusion protein obtained in step (i); (iii) providing a solution of said soluble fusion protein obtained in step (ii) in a liquid medium, (iv) adding to said liquid medium a suitable cleaving agent for achieving cleavage of the fusion protein at the cleavage agent recognition site, and thereby obtaining the major ampullate spidroin protein; and optionally (v) isolating the major ampullate spidroin protein obtained in step (iv) from said liquid medium. Further optionally, said step (ii) of isolating the soluble fusion protein, and optionally step (v) of isolating the major ampullate spidroin protein, involve(s) removal of LPS and other pyrogens.

The present invention further provides a method of producing a polymer of a major ampullate spidroin protein according to the invention, comprising the steps of: (i) expressing a polynucleic acid molecule encoding a soluble fusion protein according to the invention in a suitable host; (ii) isolating the soluble fusion protein obtained in step (i); (iii) providing a solution of said soluble fusion protein obtained in step (ii) in a liquid medium, (iv) adding to said liquid medium a suitable cleaving agent for achieving cleavage of the fusion protein at the cleavage agent recognition site, and thereby obtaining the major ampullate spidroin protein; (v) allowing the major ampullate spidroin protein obtained in step (iv) to polymerize in the liquid medium; and optionally (vi) isolating the polymer obtained in step (v) from said liquid medium. In a preferred method, said step (v) further comprises providing an interface between said liquid medium and another phase selected from the group consisting of a gas phase, a liquid phase and a solid phase, wherein said polymerizing initiates at said interface or in a region surrounding said interface. In a preferred method, said liquid medium is an aqueous medium and said other phase is selected from the group consisting of air and water-immiscible organic solvents.

According to another aspect, the present invention provides a polymer of a major ampullate spidroin protein according to the invention. The present invention also provides a polymer of a major ampullate spidroin protein obtainable by a method according to the invention. In a preferred embodiment, said polymer is a fiber. In other preferred embodiments, said polymer forms a structure selected from the group consisting of a foam, a gel, a mesh or a film.

According to yet another aspect, the present invention provides a novel use of a protein fragment comprising a fusion partner and a cleavage agent recognition site for the manufacture of a fusion protein comprising said protein fragment coupled via said cleavage agent recognition site to a spider silk protein fragment. In preferred embodiments, said spider silk protein fragment consists of from 150 to 420 amino acid residues.

According to a final aspect, the present invention provides an isolated polynucleic acid molecule comprising a nucleic acid sequence selected from the group consisting of SEQ ID NO: 1 and nucleic acid sequences encoding SEQ ID NOS: 2-16, or its complementary nucleic acid sequences. The present invention also provides use of the isolated polynucleic acid molecule for the manufacture of a non-natural gene encoding a spider silk protein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an alignment of the segments within the repetitive part of Euprosthenops australis MaSp1 protein, i.e. SEQ ID NO: 3.

FIG. 2A illustrates a schematic, predicted structural organization of the repetitive part of Euprosthenops australis MaSp1 protein (SEQ ID NO: 3). The various peptide segments shown correspond to SEQ ID NOS: 21-23 and 5-6, respectively.

FIG. 2B illustrates schematic, predicted structural organizations of the spidroin proteins constructed according to examples 5-8 (SEQ ID NOS: 9-13).

FIG. 3 is an alignment of C-terminal regions of MaSp1 and MaSp2, illustrating their conserved nature (SEQ ID NOS: 24-55).

FIG. 4 illustrates macroscopic appearances of fibers formed from spidroin proteins constructed according to examples 5-8. (A): 6Gly/Ala-CT_(hyb) protein (SEQ ID NO: 13) fibers, bar 0.5 cm; (B): 5Gly/Ala-CT_(nat) (SEQ ID NO: 9) protein fibers, bar 1 cm. (C): 5Gly/Ala-CT_(nat) (SEQ ID NO: 9) protein fibers, bar 1 cm.

FIG. 5 shows scanning electron microscopy (SEM) micrographs of fibers formed from spidroin proteins constructed according to examples 5-8. Single fibers (a) and gel-phase (b, c) from 6Gly/Ala-CT_(hyb) (SEQ ID NO: 13). Fibers of 5Gly/Ala-CT_(nat) (SEQ ID NO: 9), drawn in 75% methanol, air-dried and applied on SEM-stubs (d, e, f). Fiber twisted before air-drying (e), end of fiber (f).

FIG. 6 displays a circular dichroism (CD) spectrum of 6Gly/Ala-CT_(hyb) (SEQ ID NO: 13) fiber.

FIG. 7 illustrates the results from a mouse mast cell toxicity study, showing the numbers of live and dead cells after three days of culture in the presence or absence of in vitro produced silk fibers.

FIG. 8 is a picture of HEK293 cells following exposure to in vitro produced silk fibers in a biocompatibility study.

FIG. 9 is a stress-strain curve displaying the tensile strength of double drawn fibers from 5Gly/Ala-CT_(nat) (SEQ ID NO: 9).

FIG. 10 shows SEM micrographs of recombinant fibers from 5Gly/Ala-CT_(nat) (SEQ ID NO: 9). a,b, Spontaneously formed fibers. The close-up image (b) shows the fibrillar substructure. The small fibril that bulges out (arrow) has a width of about 300 nm. c-f, Fibers after two stretching-relaxation cycles. c and d shows the same fiber at different magnifications. e shows a cut fiber end, and f shows a point of breakage after tensile testing.

DETAILED DISCLOSURE OF THE INVENTION

The present invention is generally based on the identification of a spidroin protein motif, which is sufficient for recombinant production of spider silk fibers. The motif is based on the deduced amino acid sequence from cloning and sequencing of a partial major spidroin 1 (MaSp1) cDNA from Euprosthenops australis. It follows that the isolated MaSp1 cDNA is useful as a starting point for construction of novel spidroin genes, such as those reported herein. The polymers which are formed from the proteins resulting from the novel spidroin cDNAs are useful for their physical properties, especially the useful combination of high strength, elasticity and light weight. They are also useful for their ability to support cell adherence and growth. The properties of dragline silk are attractive in development of new materials for medical or technical purposes. In particular, spider silks according to the invention are useful in medical devices, such as implants and medical products, such as wound closure systems, band-aids, sutures, wound dressings, and scaffolds for tissue engineering and guided cell regeneration. Spider silks according to the invention are also particularly useful for use as textile or fabric, such as in parachutes, bulletproof clothing, seat belts, etc.

The term “fiber” as used herein relates to polymers having a thickness of at least 1 μm, preferably macroscopic polymers that are visible to the human eye, i.e. having a thickness of at least 1 μm, and have a considerable extension in length compared to its thickness, preferably above 5 mm. The term “fiber” does not encompass unstructured aggregates or precipitates.

The terms “major ampullate spidroin proteins”, “spidroin proteins” are used interchangeably throughout the description and encompass all known major ampullate spidroin proteins, typically abbreviated “MaSp”, or “ADF” in the case of Araneus diadematus. These major ampullate spidroin proteins are generally of two types, 1 and 2. These terms furthermore include the new proteins according to the invention, as defined in the appended claims, and other non-natural proteins with a high degree of identity and/or similarity to the known major ampullate spidroin proteins.

The present inventors have utilized the identified spidroin protein motif for construction of novel gene constructs, coding for non-natural spidroin proteins. It has been found that a major ampullate spidroin protein consisting of from 150 to 420 amino acid residues, i.e. more than or equal to 150, preferably more than or equal to 220, preferably more than or equal to 250, and less than or equal to 420, preferably less than or equal to 380 amino acid residues, preferably less than or equal to 320 amino acid residues, preferably less than or equal to 280 amino acid residues, such as 220-360 amino acid residues, can be recombinantly produced, e.g. in bacteria or other suitable production organisms. The resulting spidroin proteins spontaneously form macroscopic silk fibers according to the invention. This is a surprising result, since the naturally occurring spidroin proteins and previously known, recombinantly produced, fiber-forming spidroin proteins are considerably longer than the proteins according to the invention. Moreover, the naturally occurring spidroin proteins and previously known, recombinantly produced, fiber-forming spidroin proteins tend to contain a large number of internal repeats and require use of spinning and/or harsh solvents for polymerization.

It is here for the first time shown that spidroin proteins can spontaneously form fibers in vitro. The data presented herein also show that only a fraction of the spidroin sequence need to be present to dictate fiber formation. Moreover, a species hybrid containing a Euprosthenops repetitive domain and a Nephila non-repetitive C-terminal domain (c.f. Example 6C) forms fibers as well, indicating that the fiber-forming potential of this motif is robust.

In its general aspect, the major ampullate spidroin protein according to the invention is defined by the formula REP-CT. The REP protein fragment and the CT protein fragment are covalently coupled, typically via a peptide bond.

The protein fragment REP has a repetitive character, alternating between alanine-rich stretches and glycine-rich stretches. The REP fragment generally contains more than 80, such as more than 140, and less than 300, preferably less than 240, such as less than 200, amino acid residues, and can itself be divided into several L (linker) segments, A (alanine-rich) segments and G (glycine-rich) segments, as will be explained in more detail below. Typically, said linker segments, which are optional, are located at the REP fragment terminals, while the remaining segments are in turn alanine-rich and glycine-rich. Thus, the REP fragment can generally have either of the following structures, wherein n is an integer:

L(AG)_(n)L, such as (SEQ ID NO: 17) LA₁G₁A₂G₂A₃G₃A₄G₄A₅G₅L; (SEQ ID NO: 56) L(AG)_(n)AL, such as (SEQ ID NO: 18) LA₁G₁A₂G₂A₃G₃A₄G₄A₅G₅A₆L; (SEQ ID NO: 57) L(GA)_(n)L, such as (SEQ ID NO: 19) LG₁A₁G₂A₂G₃A₃G₄A₄G₅A₅L; or (SEQ ID NO: 58) L(GA)_(n)GL, such as (SEQ ID NO: 20) LG₁A₁G₂A₂G₃A₃G₄A₄G₅A₅G₆L. (SEQ ID NO: 59) It follows that it is not critical whether an alanine-rich or a glycine-rich segment is adjacent to the N-terminal or C-terminal linker segments. It is preferred that n is an integer from 4 to 8, more preferred from 4 to 6, i.e. n=4, n=5 or n=6.

In preferred embodiments, the alanine content of the REP fragment according to the invention is above 20%, preferably above 25%, more preferably above 30%, and below 50%, preferably below 40%, more preferably below 35%. This is advantageous, since it is contemplated that a higher alanine content provides a stiffer and/or stronger and/or less extendible fiber. The reason for this is likely to be that a higher alanine content is associated with a higher content of β-sheet structures in the fiber. Thus, in a preferred embodiment, the β-sheet content in a polymer, such as a fiber, of the major ampullate spidroin protein according to the invention is above 50%, i.e. more than 50% of the secondary structure of the protein is in β-sheet form.

In certain embodiments, the REP fragment is void of proline residues, i.e. there are no Pro residues in the REP fragment.

Now turning to the segments that constitute the REP fragment according to the invention, it shall be emphasized that each segment is individual, i.e. any two A segments, any two G segments or any two L segments of a specific REP fragment may be identical or may not be identical. Thus, it is not a general feature of the invention that each type of segment is identical within a specific REP fragment. Rather, the following disclosure provides the skilled person with guidelines how to design individual segments and gather them into a REP fragment, which is a part of a functional spidroin protein according to the invention.

It has been concluded from experimental data presented herein that each individual A segment is an amino acid sequence having from 8 to 18 amino acid residues. It is preferred that each individual A segment contains from 13 to 15 amino acid residues. It is also possible that a majority, or more than two, of the A segments contain from 13 to 15 amino acid residues, and that a minority, such as one or two, of the A segments contain from 8 to 18 amino acid residues, such as 8-12 or 16-18 amino acid residues. A vast majority of these amino acid residues are alanine residues. More specifically, from 0 to 3 of the amino acid residues are not alanine residues, and the remaining amino acid residues are alanine residues. Thus, all amino acid residues in each individual A segment are alanine residues, with no exception or the exception of one, two or three amino acid residues, which can be any amino acid. It is preferred that the alanine-replacing amino acid(s) is (are) natural amino acids, preferably individually selected from the group of serine, glutamic acid, cysteine and glycine, more preferably serine. Of course, it is possible that one or more of the A segments are all-alanine segments, while the remaining A segments contain 1-3 non-alanine residues, such as serine, glutamic acid, cysteine or glycine.

In a preferred embodiment, each A segment contains 13-15 amino acid residues, including 10-15 alanine residues and 0-3 non-alanine residues as described above. In a more preferred embodiment, each A segment contains 13-15 amino acid residues, including 12-15 alanine residues and 0-1 non-alanine residues as described above.

It is preferred that each individual A segment has at least 80% identity to an amino acid sequence selected from the group of amino acid residues 7-19, 43-56, 71-83, 107-120, 135-147, 171-183, 198-211, 235-248, 266-279, 294-306, 330-342, 357-370, 394-406, 421-434, 458-470, 489-502, 517-529, 553-566, 581-594, 618-630, 648-661, 676-688, 712-725, 740-752, 776-789, 804-816, 840-853, 868-880, 904-917, 932-945, 969-981, 999-1013, 1028-1042 and 1060-1073 of SEQ ID NO: 3. Each sequence of this group corresponds to a segment of the naturally occurring sequence of Euprosthenops australis MaSp1 protein, which is deduced from cloning of the corresponding cDNA, see Examples 1-2 and FIGS. 1-2A. Alternatively, each individual A segment has at least 80% identity to an amino acid sequence selected from the group of amino acid residues 31-42, 61-75, 90-104, 122-135 and 153-171 of SEQ ID NO: 9, amino acid residues 12-25, 46-60, 75-88, 112-119, 150-158 and 173-180 of SEQ ID NO: 13, amino acid residues 31-42 of SEQ ID NO: 14 and amino acid residues 122-135 of SEQ ID NO: 15. Each sequence of this group corresponds to a segment of expressed, non-natural spidroin proteins according to the invention, which proteins have capacity to form silk fibers under appropriate conditions. See Examples 5-8, 12 and FIG. 2B. Without wishing to be bound by any particular theory, it is envisaged that A segments according to the invention form helical structures or beta sheets.

The term “% identity”, as used throughout the specification and the appended claims, is calculated as follows. The query sequence is aligned to the target sequence using the CLUSTAL W algorithm (Thompson, J. D., Higgins, D. G. and Gibson, T. J., Nucleic Acids Research, 22: 4673-4680 (1994)). The amino acid residues at each position are compared, and the percentage of positions in the query sequence that have identical correspondences in the target sequence is reported as % identity.

The term “% similarity”, as used throughout the specification and the appended claims, is calculated as described for “% identity”, with the exception that the hydrophobic residues Ala, Val, Phe, Pro, Leu, Ile, Trp, Met and Cys are similar; the basic residues Lys, Arg and His are similar; the acidic residues Glu and Asp are similar; and the hydrophilic, uncharged residues Gln, Asn, Ser, Thr and Tyr are similar. The remaining natural amino acid Gly is not similar to any other amino acid in this context.

Throughout this description, alternative embodiments according to the invention fulfill, instead of the specified percentage of identity, the corresponding percentage of similarity. Other alternative embodiments fulfill the specified percentage of identity as well as another, higher percentage of similarity, selected from the group of preferred percentages of identity for each sequence. For example, a sequence may be 70% similar to another sequence; or it may be 70% identical to another sequence; or it may be 70% identical and 90% similar to another sequence.

In preferred embodiments according to the invention, each individual A segment has at least 90%, more preferably 95%, most preferably 100%, identity to an amino acid sequence selected from the group of amino acid residues 7-19, 43-56, 71-83, 107-120, 135-147, 171-183, 198-211, 235-248, 266-279, 294-306, 330-342, 357-370, 394-406, 421-434, 458-470, 489-502, 517-529, 553-566, 581-594, 618-630, 648-661, 676-688, 712-725, 740-752, 776-789, 804-816, 840-853, 868-880, 904-917, 932-945, 969-981, 999-1013, 1028-1042 and 1060-1073 of SEQ ID NO: 3; amino acid residues 31-42, 61-75, 90-104, 122-135 and 153-171 of SEQ ID NO: 9; amino acid residues 12-25, 46-60, 75-88, 112-119, 150-158 and 173-180 of SEQ ID NO: 13; amino acid residues 31-42 of SEQ ID NO: 14; and amino acid residues 122-135 of SEQ ID NO: 15. Thus, in certain embodiments according to the invention, each individual A segment is identical to an amino acid sequence selected from the above-mentioned amino acid segments.

Furthermore, it has been concluded from experimental data presented herein that each individual G segment is an amino acid sequence of from 12 to 30 amino acid residues. It is preferred that each individual G segment consists of from 14 to 23 amino acid residues. At least 40% of the amino acid residues of each G segment are glycine residues. Typically the glycine content of each individual G segment is in the range of 40-60%.

It is preferred that each individual G segment has at least 80% identity to an amino acid sequence selected from the group of amino acid residues 20-42, 57-70, 84-106, 121-134, 148-170, 184-197, 212-234, 249-265, 280-293, 307-329, 343-356, 371-393, 407-420, 435-457, 471-488, 503-516, 530-552, 567-580, 595-617, 631-647, 662-675, 689-711, 726-739, 753-775, 790-803, 817-839, 854-867, 881-903, 918-931, 946-968, 982-998, 1014-1027, 1043-1059 and 1074-1092 of SEQ ID NO: 3. Each sequence of this group corresponds to a segment of the naturally occurring sequence of Euprosthenops australis MaSp1 protein, which is deduced from cloning of the corresponding cDNA, see Examples 1-2 and FIGS. 1-2A. Alternatively, each individual G segment has at least 80% identity to an amino acid sequence selected from the group of amino acid residues 11-30, 43-60, 76-89, 105-121 and 136-152 of SEQ ID NO: 9 and amino acid residues 1-11, 26-45, 61-74, 89-111, 120-149 and 159-172 of SEQ ID NO: 13. Each sequence of this group corresponds to a segment of expressed, non-natural spidroin proteins according to the invention, which proteins have capacity to form silk fibers under appropriate conditions. See Examples 5-8, 12 and FIG. 2B.

In preferred embodiments according to the invention, each individual G segment has at least 90%, more preferably 95%, most preferably 100%, identity to an amino acid sequence selected from the group of amino acid residues 20-42, 57-70, 84-106, 121-134, 148-170, 184-197, 212-234, 249-265, 280-293, 307-329, 343-356, 371-393, 407-420, 435-457, 471-488, 503-516, 530-552, 567-580, 595-617, 631-647, 662-675, 689-711, 726-739, 753-775, 790-803, 817-839, 854-867, 881-903, 918-931, 946-968, 982-998, 1014-1027, 1043-1059 and 1074-1092 of SEQ ID NO: 3; amino acid residues 11-30, 43-60, 76-89, 105-121 and 136-152 of SEQ ID NO: 9; and amino acid residues 1-11, 26-45, 61-74, 89-111, 120-149 and 159-172 of SEQ ID NO: 13. Thus, in certain embodiments according to the invention, each individual G segment is identical to an amino acid sequence selected from the above-mentioned amino acid segments.

In certain embodiments, the first two amino acid residues of each G segment according to the invention are not -Gln-Gln-.

In certain embodiments, the position corresponding to the conserved Tyr residue (i.e. corresponding to position 16 in SEQ ID NO: 5, position 10 in SEQ ID NO: 6 and position 7 in SEQ ID NO: 7) is not Phe in any G segment according to the invention.

In certain embodiments, the position corresponding to the conserved Tyr residue (i.e. corresponding to position 16 in SEQ ID NO: 5, position 10 in SEQ ID NO: 6 and position 7 in SEQ ID NO: 7) is Tyr in each G segment according to the invention.

It follows that certain embodiments of the proteins according to the invention display a combination of the above-mentioned limitations, i.e. the first two amino acid residues of each G segment according to the invention are not -Gln-Gln-, and the conserved Tyr residue (i.e. corresponding to position 16 in SEQ ID NO: 5, position 10 in SEQ ID NO: 6 and position 7 in SEQ ID NO: 7) is Tyr in each G segment according to the invention. In certain embodiments, the above-mentioned limitations, taken separately or in any possible combination, can be further combined with the limitation that the REP fragment is void of proline residues, as discussed hereinabove.

With reference to FIGS. 1-2 and Examples 3-4, there are the three subtypes of the G segment according to the invention. This classification is based upon careful analysis of the Euprosthenops australis MaSp1 protein sequence (FIGS. 1-2A), and the information has been employed and verified in the construction of novel, non-natural spider silk proteins (FIG. 2B).

The first subtype of the G segment according to the invention is represented by the amino acid one letter consensus sequence GQG(G/S)QGG(Q/Y)GG (L/Q)GQGGYGQGA GSS, as shown in FIG. 2A and SEQ ID NO: 5. This first, and generally the longest, G segment subtype typically contains 23 amino acid residues, but may contain as little as 17 amino acid residues, and lacks charged residues or contain one charged residue. Thus, it is preferred that this first G segment subtype contains 17-23 amino acid residues, but it is contemplated that it may contain as few as 12 or as many as 30 amino acid residues. Without wishing to be bound by any particular theory, it is envisaged that this subtype forms coil structures or 3₁-helix structures. Representative G segments of this first subtype are amino acid residues 20-42, 84-106, 148-170, 212-234, 307-329, 371-393, 435-457, 530-552, 595-617, 689-711, 753-775, 817-839, 881-903, 946-968, 1043-1059 and 1074-1092 of SEQ ID NO: 3; amino acid residues 11-30, 105-121 and 136-152 of SEQ ID NO: 9; and amino acid residues 26-45 and 89-111 of SEQ ID NO: 13. Alternative G segments of this first subtype are amino acid residues 120-149 and 159-172 of SEQ ID NO: 13. In certain embodiments, the first two amino acid residues of each G segment of this first subtype according to the invention are not -Gln-Gln-.

The second subtype of the G segment according to the invention is represented by the amino acid one letter consensus sequence GQGGQGQG(G/R)Y GQG(A/S)G(S/G)S, as shown in FIG. 2A and SEQ ID NO: 6. This second, generally mid-sized, G segment subtype typically contains 17 amino acid residues and lacks charged residues or contain one charged residue. It is preferred that this second G segment subtype contains 14-20 amino acid residues, but it is contemplated that it may contain as few as 12 or as many as 30 amino acid residues. Without wishing to be bound by any particular theory, it is envisaged that this subtype forms coil structures. Representative G segments of this second subtype are amino acid residues 249-265, 471-488, 631-647 and 982-998 of SEQ ID NO: 3; and amino acid residues 43-60 of SEQ ID NO: 9.

The third subtype of the G segment according to the invention is represented by the amino acid one letter consensus sequence G(R/Q)GQG(G/R)YGQG (A/S/V)GGN, as shown in FIG. 2A and SEQ ID NO: 7. This third G segment subtype typically contains 14 amino acid residues, and is generally the shortest of the G segment subtypes according to the invention. It is preferred that this third G segment subtype contains 12-17 amino acid residues, but it is contemplated that it may contain as many as 23 amino acid residues. Without wishing to be bound by any particular theory, it is envisaged that this subtype forms turn structures. Representative G segments of this third subtype are amino acid residues 57-70, 121-134, 184-197, 280-293, 343-356, 407-420, 503-516, 567-580, 662-675, 726-739, 790-803, 854-867, 918-931, 1014-1027 of SEQ ID NO: 3; amino acid residues 76-89 of SEQ ID NO: 9; and amino acid residues 61-74 of SEQ ID NO: 13. An alternative G segment of this third subtype is amino acid residues 1-11 of SEQ ID NO: 13.

Thus, in preferred embodiments, each individual G segment has at least 80%, preferably 90%, more preferably 95%, identity to an amino acid sequence selected from SEQ ID NO: 5, SEQ ID NO: 6 and SEQ ID NO: 7.

In a preferred embodiment of the alternating sequence of A and G segments of the REP fragment, every second G segment is of the first subtype, while the remaining G segments are of the third subtype, e.g. . . . A₁G_(short)A₂G_(long)A₃G_(short)A₄G_(long)A₅G_(short) . . . (SEQ ID NO: 56) In another preferred embodiment of the REP fragment, one G segment of the second subtype interrupts the G segment regularity via an insertion, e.g. . . . A₁G_(short)A₂G_(long)A₃G_(mid)A₄G_(short)A₅G_(long) . . . (SEQ ID NO: 56)

Each individual L segment represents an optional linker amino acid sequence, which may contain from 0 to 20 amino acid residues, such as from 0 to 10 amino acid residues. While this segment is optional and not functionally critical for the spidroin protein, its presence still allows for fully functional spidroin proteins, forming spider silk fibers according to the invention. There are also linker amino acid sequences present in the repetitive part (SEQ ID NO: 3) of the deduced amino acid sequence of the MaSp1 protein from Euprosthenops australis. In particular, the amino acid sequence of a linker segment may resemble any of the described A or G segments, but usually not sufficiently to meet their criteria as defined herein.

As shown in FIG. 2A, a linker segment arranged at the C-terminal part of the REP fragment can be represented by the amino acid one letter consensus sequences ASASAAASAA STVANSVS (SEQ ID NO: 60) and ASAASAAA (SEQ ID NO: 61), which are rich in alanine. In fact, the second sequence can be considered to be an A segment according to the invention, while the first sequence has a high degree of similarity to A segments according to the invention. Another example of a linker segment according the invention has the one letter amino acid sequence GSAMGQGS (SEQ ID NO: 62), which is rich in glycine and has a high degree of similarity to G segments according to the invention.

Representative L segments are amino acid residues 1-6 and 1093-1110 of SEQ ID NO: 3; amino acid residues 1-10 and 153-171 of SEQ ID NO: 9; and amino acid residues 173-180 of SEQ ID NO: 13, but the skilled person in the art will readily recognize that there are many suitable alternative amino acid sequences for these segments. In one embodiment of the REP fragment according to the invention, one of the L segments contains 0 amino acids, i.e. one of the L segments is void. In another embodiment of the REP fragment according to the invention, both L segments contain 0 amino acids, i.e. both L segments are void. Thus, these embodiments of the REP fragments according to the invention may be schematically represented as follows: (AG)_(n)L, (AG)_(n)AL, (GA)_(n)L, (GA)_(n)GL; L(AG)_(n), L(AG)_(n)A, L(GA)_(n), L(GA)_(n)G; and (AG)_(n), (AG)_(n)A, (GA)_(n), (GA)_(n)G. Any of these REP fragments are suitable for use with any CT fragment as defined below.

The C-terminal (CT) fragment of the spidroin protein according to the invention has a high degree of similarity to the C-terminal amino acid sequence of spidroin proteins. As shown in FIG. 3, this amino acid sequence is well conserved among various species and spidroin proteins, including MaSp1 and MaSp2. It is demonstrated in the following examples that it is not critical which specific CT fragment is present in spidroin proteins according to the invention, as long as the CT fragment is not entirely missing. Thus, the CT fragment according to the invention can be selected from any of the amino acid sequences shown in FIG. 3 or sequences with a high degree of similarity. It is notable that the CT_(hyb) fragment of SEQ ID NO: 13 has 96% identity to the consensus amino acid sequence SEQ ID NO: 8, while the CT_(nat) fragment of SEQ ID NO: 9 displays only 59% identity to the consensus amino acid sequence SEQ ID NO: 8. This illustrates that a wide variety of C-terminal sequences can be used in the spidroin protein according to the invention.

The sequence of the CT fragment according to the invention has at least 50% identity, preferably at least 60% identity, to the consensus amino acid sequence SEQ ID NO: 8, which is based on the amino acid sequences of FIG. 3. In a preferred embodiment, the sequence of the CT fragment according to the invention has at least 65% identity, preferably at least 70% identity, to amino acid residues 1-71 of the consensus amino acid sequence SEQ ID NO: 8. In preferred embodiments, the CT fragment according to the invention has furthermore 70%, preferably 80%, similarity to the consensus amino acid sequence SEQ ID NO: 8, or amino acid residues 1-71 thereof.

Representative CT fragments according to the invention are the Euprosthenops australis sequence SEQ ID NO: 4, the Euprosthenops australis-derived amino acid residues 172-269 of SEQ ID NO: 9 and amino acid residues 181-276 of SEQ ID NO: 13, alleged to be derived from Euprosthenops sp (Pouchkina-Stantcheva, N. N. & McQueen-Mason, S. J. Molecular studies of a novel dragline silk from a nursery web spider, Euprosthenops sp. (Pisauridae). Comp Biochem Physiol B Biochem Mol Biol 138, 371-376 (2004)), but with a high degree of similarity to MaSp1 from Nephila clavipes and Nephila senegalensis. Thus, according to a preferred aspect of the invention, the CT fragment has at least 80% identity to SEQ ID NO: 4, amino acid residues 172-269 of SEQ ID NO: 9, amino acid residues 181-276 of SEQ ID NO: 13, amino acid residues 172-269 of SEQ ID NO: 16 or any individual MaSp1/ADF-4 amino acid sequence of FIG. 3 and Example 4. In preferred aspects of the invention, the CT fragment has at least 90%, such as at least 95% identity, to SEQ ID NO: 4, amino acid residues 172-269 of SEQ ID NO: 9, amino acid residues 181-276 of SEQ ID NO: 13, amino acid residues 172-269 of SEQ ID NO: 16 or any individual MaSp1/ADF-4 amino acid sequence of FIG. 3 and Example 4. In preferred aspects of the invention, the CT fragment is identical to SEQ ID NO: 4, amino acid residues 172-269 of SEQ ID NO: 9, amino acid residues 181-276 of SEQ ID NO: 13, amino acid residues 172-269 of SEQ ID NO: 16 or any individual MaSp1/ADF-4 amino acid sequence of FIG. 3 and Example 4.

The CT fragment typically consists of from 70 to 120 amino acid residues. It is preferred that the CT fragment contains at least 70, or more than 80, preferably more than 90, amino acid residues. It is also preferred that the CT fragment contains at most 120, or less than 110 amino acid residues. A typical CT fragment contains approximately 100 amino acid residues.

According to another aspect, the present invention provides an isolated fusion protein consisting of a first protein fragment, which is a major ampullate spidroin protein, preferably consisting of from 150 to 420 amino acid residues, and a second protein fragment, which comprises a fusion partner and a cleavage agent recognition site. The first protein fragment is coupled via the cleavage agent recognition site to the fusion partner, i.e. the fusion partner can be cleaved off by treating the fusion protein with a suitable cleaving agent under appropriate conditions, providing a major ampullate spidroin protein, preferably consisting of from 150 to 420 amino acid residues. An advantage with this fusion protein is that large amounts thereof can be produced in solution, preferably in a physiological medium, typically a buffered aqueous medium, such as a 10-100 mM Tris-HCl buffer, pH 6-9, without causing precipitation and other production problems when produced in suitable hosts, such as bacteria, preferably E. coli. The fusion proteins in the solution are soluble for long time periods, typically days or weeks, which facilitates large-scale production and decreases the risk for protein aggregation. By the terms “soluble” and “in solution” is meant that the protein is not visibly aggregated and does not precipitate from the solvent at 60 000×g. At wish, the fusion proteins in the solution can be subjected to cleavage using a suitable cleaving agent, providing a major ampullate spidroin protein which spontaneously forms silk fibers.

In a preferred aspect, the present invention provides an isolated fusion protein selected from the group of X-REP-CT and REP-CT-X, preferably X-REP-CT. REP and CT are protein fragments according to the invention, implying that the resulting MaSp protein of the form REP-CT is a MaSp protein according to the invention. X is a protein fragment comprising a fusion partner and a cleavage agent recognition site as defined above. The combined protein fragment REP-CT is coupled via the cleavage agent recognition site to the fusion partner.

Fusion partners according to the invention include any protein fragment which improves the solubility and/or stability of its partner protein fragment, here the MaSp protein according to the invention. The fusion partner also provides a suitable handle for affinity purification. Without being limited thereto, examples of fusion partners according to the invention include thioredoxin, maltose-binding protein, glutathione S-transferase (GST), MTB32-C, Gb1, ZZ and Nus A. The skilled person is well aware of alternative suitable fusion partners. In a preferred embodiment of the invention, the fusion partner is a thioredoxin moiety (ThrX) in combination with a His tag and an S tag. In one preferred embodiment of the invention, the fusion partner is a ThrX moiety in combination with two His tags, i.e. His-tag/ThrX/His-tag. In another preferred embodiment of the invention, the fusion partner is a thioredoxin moiety (ThrX).

The cleavage agent recognition site is situated at that X protein fragment terminal which is coupled to the MaSp protein fragment, so that cleavage at the recognition site results in a MaSp protein and a fusion partner. Without being limited thereto, examples of the cleavage agent recognition site according to the invention include a thrombin recognition site having the amino acid sequence LVPRGS (SEQ ID NO: 63) (cleaves between R and G); an enterokinase recognition site having the amino acid sequence DDDK (SEQ ID NO: 64) (cleaves after K); an hydroxylamine recognition site having the amino acid sequence NG (cleaves between N and G); a HRV 3C protease recognition site having the amino acid sequence LGVLFQGP (SEQ ID NO: 65)(cleaves between Q and G); a Factor Xa recognition site having the amino acid sequence I(E/D)GR (SEQ ID NO: 66) (cleaves after R); a TEV recognition site having the amino acid sequence EXXYXQ(G/S) (SEQ ID NO: 67), commonly ENLYFQG (SEQ ID NO: 68)(cleaves between Q and G/S), an Ac-TEV recognition site having the amino acid sequence EDNLYFQG (SEQ ID NO: 69)(cleaves between Q and G); and a PreScission recognition site having the amino acid sequence LEVLFQGP (SEQ ID NO: 70) (cleaves between Q and G). Other suitable recognition sites are the cleavage sites for trypsin, endoproteinase, V8 protease, pepsin and CNBr. Further examples of suitable cleavage recognition sites are well within the reach of the skilled person. In a preferred embodiment of the invention, the cleavage agent recognition site is a thrombin recognition site.

In a preferred embodiment, the X fragment according to the invention has the structure ThrX/His-tag/S-tag/thrombin cleavage site, and the X fragment is coupled to the N-terminal of the REP-CT protein fragment according to the invention.

In one preferred embodiment, the X fragment according to the invention has the structure His-tag/ThrX/His-tag/thrombin cleavage site, and the X fragment is coupled to the N-terminal of the REP-CT protein fragment according to the invention.

According to another aspect, the present invention provides a method of producing a major ampullate spidroin protein according to the invention. In the first step, a solution of a fusion protein according to the invention in a liquid medium is provided. Preferably, the fusion protein does not aggregate, and therefore, resolubilization procedures are not required. The fusion protein can be recombinantly produced and purified using an affinity handle at the fusion protein, such as a His-tag or any suitable epitope in the fusion protein. The liquid medium can be any suitable medium, preferably a physiological medium, typically a buffered aqueous medium, such as a 10-100 mM Tris-HCl buffer, pH 6-9. In the second step, a cleavage agent according to the invention is added to the liquid medium in order to achieve cleavage of the fusion protein at the cleavage agent recognition site. As disclosed above, the major ampullate spidroin protein according to the invention is thereby obtained. In a third, optional step, the thus obtained major ampullate spidroin protein is isolated from the liquid medium using suitable isolation techniques, such as chromatography and/or filtration.

According to yet another aspect, the present invention provides a method of producing a polymer of a major ampullate spidroin protein according to the invention. In the first step, a solution of a fusion protein according to the invention in a liquid medium is provided. Preferably, the fusion protein does not aggregate, and therefore, resolubilization procedures are not required. The fusion protein can be recombinantly produced and purified using an affinity handle at the fusion protein, such as a His-tag or any suitable epitope in the fusion protein. The liquid medium can be any suitable medium, preferably a physiological medium, typically a buffered aqueous medium, such as a 10-100 mM Tris-HCl buffer, pH 6-9. In the second step, a cleavage agent according to the invention is added to the liquid medium in order to achieve cleavage of the fusion protein at the cleavage agent recognition site. As disclosed above, the major ampullate spidroin protein according to the invention is thereby obtained. In the third step, the thus obtained major ampullate spidroin protein is allowed to polymerize in the liquid medium. The polymerization typically initiates at the interface between two different phases, such as liquid/air, liquid/solid, and water/oil interfaces. Thus, this third step may also further comprise providing an interface between the liquid medium and another phase. The other phase is selected from the group consisting of a gas phase, a liquid phase and a solid phase. As detailed above, the liquid medium is typically an aqueous medium, and suitable other phases are for instance air and water-immiscible organic solvents, such as oil, e.g. mineral oil suitable for PCR reactions. The presence of the resulting interface stimulates polymerization at the interface or in the region surrounding the interface, which region extends into the liquid medium, such that said polymerizing initiates at said interface or in said interface region. Preferred interfaces include water/air and water/oil interfaces. Polymerization typically occurs spontaneously within minutes or a few hours, such as within from 1 min to 5 h, upon incubation at room temperature. In a fourth, optional step, the thus obtained polymer of the major ampullate spidroin protein is isolated from the liquid medium using suitable isolation techniques.

As discussed above, fiber formation is induced by proteolytic release of the miniature spidroin from the fusion protein. If the cleavage reaction is performed in a tube that is gently wagged from side to side, a fiber is formed at the air-water interface along the tube. The tube can be made of any suitable material, such as plastic or glass. If the cleavage mixture is allowed to stand still, a film is formed at the air-water interface. If oil is added on top of the aqueous cleavage mixture, a film is formed at the oil-water interface, either if allowed to stand still or if wagged. If the cleavage mixture is foamed, e.g. by bubbling of air or whipping, the foam is stable and solidifies if allowed to dry.

Using the method(s) of the present invention, it is possible to recombinantly produce large amounts of fusion proteins according the invention, which can be cleaved and allowed to polymerize at desire. This provides a better control of the polymerization process and allows for optimization of parameters for obtaining silk fibers with desirable properties.

The major ampullate spidroin protein according to the invention is typically recombinantly produced using a variety of suitable hosts. According to another aspect, the present invention therefore provides an isolated polynucleic acid molecule comprising a nucleic acid sequence which encodes a major ampullate spidroin protein according to the invention, or its complementary nucleic acid sequence, such as SEQ ID NOS: 9-13, preferably SEQ ID NOS: 9, 12 and 13. These polynucleic acid molecules as well as polynucleic acid molecules coding for the various proteins disclosed herein (SEQ ID NOS: 2-16) may also be useful in further developments of non-natural spidroin proteins or production systems therefor.

The fusion protein according to the invention is typically recombinantly produced using a variety of suitable hosts, such as bacteria, yeast, mammalian cells, plants, insect cells, and transgenic animals. It is preferred that the fusion protein according to the invention is produced in bacteria.

According to another aspect, the present invention therefore provides an isolated polynucleic acid molecule comprising a nucleic acid sequence which encodes a fusion protein according to the invention, or its complementary nucleic acid sequence. The polynucleic acid molecule may also be useful in further developments of non-natural spidroin proteins or production systems therefor.

Polynucleic acid molecules according to the invention can be DNA molecules, including cDNA molecules, or RNA molecules. As the skilled person is well aware, a nucleic acid sequence may as well be described by its complementary nucleic acid sequence. Therefore, nucleic acid sequences that are complementary to the nucleic acid sequences according to the invention are also encompassed by the protective scope of the invention.

According to one aspect, the present invention provides a method of producing a soluble fusion protein according to the invention. In the first step, a polynucleic acid molecule which encodes a fusion protein according to the invention is expressed in a suitable host. In the second step, the thus obtained soluble fusion protein in step is isolated, e.g. using chromatography and/or filtration. Optionally, said second step of isolating the soluble fusion protein involves removal of LPS and other pyrogens.

The present invention further provides a method of producing a major ampullate spidroin protein according to the invention. In the first step, a polynucleic acid molecule which encodes a fusion protein according to the invention is expressed in a suitable host. In the second step, the thus obtained soluble fusion protein is isolated, e.g. using chromatography and/or filtration. In the third step, a solution of the isolated fusion protein is provided, and in the fourth step, a suitable cleaving agent is added to the liquid medium. This achieves cleavage of the fusion protein at the cleavage agent recognition site, and thereby provides the major ampullate spidroin protein. In an optional fifth step, the thus obtained major ampullate spidroin protein is isolated from the liquid medium. Further optionally, said second step of isolating the soluble fusion protein, and optionally the fifth step of isolating the major ampullate spidroin protein, involve(s) removal of LPS and other pyrogens.

The present invention also provides a method of producing a polymer of a major ampullate spidroin protein according to the invention. In the first step, a polynucleic acid molecule which encodes a fusion protein according to the invention is expressed in a suitable host. In the second step, the thus obtained soluble fusion protein is isolated, e.g. using chromatography and/or filtration. In the third step, a solution of the isolated fusion protein is provided, and in the fourth step, a suitable cleaving agent is added to the liquid medium. This achieves cleavage of the fusion protein at the cleavage agent recognition site, and thereby provides the major ampullate spidroin protein. In the fifth step, the thus obtained major ampullate spidroin protein is allowed to polymerize in the liquid medium. The polymerization typically initiates at the interface between two different phases, such as liquid/air, liquid/solid, and water/oil interfaces. Thus, this fifth step may also further comprise providing an interface between the liquid medium and another phase. The other phase is selected from the group consisting of a gas phase, a liquid phase and a solid phase. As detailed above, the liquid medium is typically an aqueous medium, and suitable other phases are for instance air and water-immiscible organic solvents, such as oil, e.g. mineral oil suitable for PCR reactions. The presence of the resulting interface stimulates polymerization at the interface or in the region surrounding the interface, which region extends into the liquid medium, such that said polymerizing initiates at said interface or in said interface region. Preferred interfaces include water/air and water/oil interfaces. Polymerization typically occurs spontaneously within minutes or a few hours, such as within from 1 min to 5 h, upon incubation at room temperature. In an optional sixth step, the thus obtained polymer is isolated from the liquid medium.

In order to obtain a protein with low pyrogenic content, which is an obligate for usage as a biomaterial in vivo, a purification protocol optimized for removal of lipopolysaccharides (LPS) has been developed. To avoid contamination by released LPS, the producing bacterial cells are subjected to washing steps with altering CaCl₂ and EDTA. After cell lysis, all subsequent purifications steps are performed in low conductivity buffers in order to minimize hydrophobic interactions between the target protein and LPS. The LPS content is further minimized by passage of the protein solution through an Endotrap column, which has a ligand that specifically adsorbs LPS. To assure constant low content of LPS and other pyrogens, all batches are analyzed using an in vitro pyrogen test (IPT) and/or a Limulus amebocyte lysate (LAL) kinetic assay. Although produced in a gram-negative bacterial host, the recombinant spidroin proteins can be purified so that residual levels of LPS and other pyrogens are below the limits required for animal tests, i.e. below 25 EU/implant. In certain embodiments according to the invention, the content of LPS and other pyrogens in the isolated fusion protein is 1 EU/mg protein or lower. In certain embodiments according to the invention, the content of LPS and other pyrogens in the isolated major ampullate spidroin protein is 1 EU/mg protein or lower, preferably 0.25 EU/mg protein or lower.

According to another aspect, the present invention provides a polymer of a major ampullate spidroin protein according to the invention. In a preferred embodiment, the polymer of this protein is obtainable by any one of the methods therefor according to the invention.

In preferred embodiments, the β-sheet content of the polymer of the major ampullate spidroin protein according to the invention is above 50%, i.e. more than 50% of the secondary structure of the polymer of this protein is in β-sheet form. This is advantageous, since it is contemplated that a higher content of β-sheet structures provides a stiffer and/or stronger and/or less extendible fiber.

It is preferable that the polymer of the spidroin protein according to the invention is a fiber with a macroscopic size, i.e. with a diameter above 1 μm, preferably above 10 μm and a length above 5 mm. It is preferred that the fiber has a diameter in the range of 10-400 μm, preferably 60-120 μm, and a length in the range of 0.5-300 cm, preferably 1-100 cm. Other preferred ranges are 0.5-30 cm and 1-20 cm. It is also preferred that the polymer of the spidroin protein according to the invention has a tensile strength above 1 MPa, preferably above 2 MPa, more preferably 10 MPa or higher. It is preferred that the polymer of the spidroin protein according to the invention has a tensile strength above 100 MPa, more preferably 200 MPa or higher. The fiber has the capacity to remain intact during physical manipulation, i.e. can be used for spinning, weaving, twisting, crocheting and similar procedures.

In other preferred embodiments, the polymer of the spidroin protein according to the invention forms a foam, a gel, a mesh or a film.

According to yet another aspect, the present invention provides a novel use of a protein fragment comprising a fusion partner and a cleavage agent recognition site for the manufacture of a fusion protein. The fusion protein is comprising said protein fragment and a spider silk protein fragment according to the invention, and the two fragments are coupled via said cleavage agent recognition site. The spider silk protein fragment preferably consists of from 150 to 420 amino acid residues.

The present invention will in the following be further illustrated by the following non-limiting examples.

EXAMPLES Example 1 Cloning and Sequencing of Euprosthenops Australis MaSp1 cDNA

The major ampullate glands from approximately 100 adult female Euprosthenops australis spiders, collected in South Africa, were used to construct a custom-made pDONR222-based CloneMiner cDNA library (Invitrogen, Paisley, UK). cDNA clones encoding the MaSp1 protein were obtained by screening the library with a cDNA probe encoding an alanine- and glycine-rich fragment originating from Euprosthenops spiders of unknown subspecies. Colony blotting and detection were performed with an ECL direct labelling and detection system (Amersham Biosciences, Uppsala, Sweden) according to the manufacturer's instruction.

One single clone was chosen for further characterization. To obtain full length sequence of the cDNA insert from this clone, nested deletions were made using the Erase-a-Base System (Promega, Southampton, UK), and sequencing was performed on a MegaBase 1000 instrument (Amersham Biosciences).

The resulting 3.8 kb cDNA (SEQ ID NO: 1) encodes a MaSp1 protein (SEQ ID NO: 2) of 1207 amino acid residues, containing a repetitive fragment of 34 alanine- and glycine-rich segments (SEQ ID NO: 3), and a C-terminal non-repetitive fragment of 97 amino acid residues (SEQ ID NO: 4).

Example 2 Sequence Analysis of the Repetitive Fragment of Euprosthenops Australis MaSp1 Protein

The repetitive fragment of the Euprosthenops australis MaSp1 protein sequence of Example 1 (SEQ ID NO: 3) was further analyzed by alignment of the repetitive segments of the fragment, see FIG. 1. The alignment was carefully scrutinized and the following structural information was concluded.

The alanine-rich segments of the Euprosthenops australis MaSp1 protein are 13-15 amino acid residues long and consists of only alanine residues or all alanine residues but one residue, which is a serine, glutamate or glycine residue.

The repetitive fragment of the Euprosthenops australis MaSp1 protein further contains three related, but distinct, types of glycine-rich segments, c.f. FIG. 2A. Two of the glycine-rich segments differ almost only in length and occurrence; the most common glycine-rich segment contains 23 amino acid residues, while a less abundant variant contains 17 amino acid residues. Both of these glycine-rich segments generally lack charged residues or contain one charged residue. In contrast, the shortest glycine-rich segment, containing 14 amino acid residues, uniquely contains the sequence GRGQG (SEQ ID NO: 71) or GQGQG (SEQ ID NO: 72) at the N-terminal end, and GN at the C-terminal end.

The longest glycine-rich segment is represented by the amino acid one letter consensus sequence GQG(G/S)QGG(Q/Y)GG (L/Q)GQGGYGQGA GSS (SEQ ID NO: 5), and lacks charged residues. It is predicted that this segment forms coil structures or 3₁-helix structures. The mid-sized glycine-rich segment is represented by the amino acid one letter consensus sequence GQGGQGQG(G/R)Y GQG(A/S)G(S/G)S (SEQ ID NO: 6), and lacks charged residues or contains one charged residue. It is predicted that this segment forms coil structures. The shortest glycine-rich segment is represented by the amino acid one letter consensus sequence G(R/Q)GQG(G/R)YGQG (A/S/V)GGN (SEQ ID NO: 7). It is predicted that this segment forms turn structures.

The repetitive fragment of the Euprosthenops australis MaSp1 protein is built up from alternating alanine-rich and glycine rich segments, e.g.

. . . A₁G₁A₂G₂A₃G₃A₄G₄A₅G₅ . . . (SEQ ID NO: 56)

It is observed that each of the above-identified shortest and longest glycine-rich segments generally occur as every second glycine-rich segment, e.g.

. . . A₁G_(short)A₂G_(long)A₃G_(short)A₄G_(long)A₅G_(short) (SEQ ID NO: 56).

In contrast, the less abundant, mid-sized glycine-rich fragment generally occurs in between a glycine-rich segment of the longer type and a glycine-rich segment of the shorter type, e.g.

. . . A₁G_(short)A₂G_(long)A₃G_(mid)A₄G_(short)A₅G_(long) . . . (SEQ ID NO: 56)

Example 3 Prediction of Secondary and Tertiary Structure Of the Repetitive Fragment of Euprosthenops Australis MaSp1 Protein

Spidroin polypeptides in solution typically fold by formation of hairpin structures, which prefigures the anti-parallel β-sheet structure of the mature fiber. To discern possible folding patterns for the repetitive fragment (SEQ ID NO: 3) of the Euprosthenops australis MaSp1 protein of examples 1-2, protein regions that are compatible with formation of hairpin or turn structures were identified. The alanine-rich segments are unlikely candidates for turn formation since they are predicted to form helical structures, and more importantly, these segments are generally held to make up the β-sheets in the fiber.

Using a recently described algorithm for turn predictions (Fuchs, PF & Alix, AJ, High accuracy prediction of beta-turns and their types using propensities and multiple alignments. Proteins 59, 828-839 (2005)), the shortest glycine-rich segments shows high likelihood for formation of type II β-turns, while the two longer glycine-rich segments are predicted to form coil structures. The high content of GGX triplets in the longer Gly-rich segments suggests that they can form 3₁-helix structures.

The repetitive nature of the spidroin amino acid sequences implies an equally repetitive nature of the folding pattern. Taken together, these observations result in a folding of the repetitive fragment of the Euprosthenops australis MaSp1 protein as shown in FIG. 2A. It is notable that the positively charged residues almost invariably are located in the proposed turn structures.

From the folding pattern of the repetitive fragment of the Euprosthenops australis MaSp1 protein, a motif consisting of alanine-rich segment/(longer) glycine-rich coil segment/alanine-rich segment/(shorter) glycine-rich turn segment/alanine-rich segment/(longer) glycine-rich coil segment/alanine-rich segment, can be discerned (schematically illustrated in FIG. 2A).

Example 4 Sequence Analysis of the Non-Repetitive C-Terminal Fragment of MaSp1 Proteins

The primary structure of the C-terminal non-repetitive fragment (SEQ ID NO: 4) of MaSp1 protein from Euprosthenops australis, obtained in Example 1, was aligned with a number of known C-terminal fragments of MaSp1 and MaSp2 proteins, inter alia from Euprosthenops sp. (Pouchkina-Stantcheva, NN & McQueen-Mason, SJ, Molecular studies of a novel dragline silk from a nursery web spider, Euprosthenops sp. (Pisauridae). Comp Biochem Physiol B Biochem Mol Biol 138, 371-376 (2004)), Nephila clavipes P19837-5 (Xu, M & Lewis, RV, Structure of a protein superfiber: spider dragline silk. Proc Natl Acad Sci USA 87, 7120-7124 (1990)) and others.

From the alignment shown in FIG. 3, starting from the last Ser in the repetitive fragment, it is evident that the C-terminal regions of MaSp1 and MaSp2 are well conserved. Euprosthenops sp and Nephila clavipes have 95% identical residues; Euprosthenops australis and Nephila clavipes have 54% identical residues; and Euprosthenops australis and Euprosthenops sp have 55% identical residues. A consensus sequence of the C-terminal regions of MaSp1 and MaSp2 is provided as SEQ ID NO: 8. In FIG. 3, the following MaSp proteins are aligned, denoted with GenBank accession entries where applicable:

Species and spidroin protein Entry Euprosthenops sp MaSp1 (Pouchkina- Cthyb_Esp Stantcheva, NN & McQueen-Mason, SJ, ibid) Euprosthenops australis MaSp1 (SEQ ID NO: 4) CTnat_Eau Argiope trifasciata MaSp1 AF350266_At1 Cyrtophora moluccensis Sp1 AY666062_Cm1 Latrodectus geometricus MaSp1 AF350273_Lg1 Latrodectus hesperus MaSp1 AY953074_Lh1 Macrothele holsti Sp1 AY666068_Mh1 Nephila clavipes MaSp1 U20329_Nc1 Nephila pilipes MaSp1 AY666076_Np1 Nephila madagascariensis MaSp1 AF350277_Nm1 Nephila senegalensis MaSp1 AF350279_Ns1 Octonoba varians Sp1 AY666057_Ov1 Psechrus sinensis Sp1 AY666064_Ps1 Tetragnatha kauaiensis MaSp1 AF350285_Tk1 Tetragnatha versicolor MaSp1 AF350286_Tv1 Araneus bicentenarius Sp2 ABU20328_Ab2 Argiope amoena MaSp2 AY365016_Aam2 Argiope aurantia MaSp2 AF350263_Aau2 Argiope trifasciata MaSp2 AF350267_At2 Gasteracantha mammosa MaSp2 AF350272_Gm2 Latrodectus geometricus MaSp2 AF350275_Lg2 Latrodectus hesperus MaSp2 AY953075_Lh2 Nephila clavipes MaSp2 AY654293_Nc2 Nephila madagascariensis MaSp2 AF350278_Nm2 Nephila senegalensis MaSp2 AF350280_Ns2 Dolomedes tenebrosus Fb1 AF350269_DtFb1 Dolomedes tenebrosus Fb2 AF350270_DtFb2 Araneus diadematus ADF-1 U47853_ADF1 Araneus diadematus ADF-2 U47854_ADF2 Araneus diadematus ADF-3 U47855_ADF3 Araneus diadematus ADF-4 U47856_ADF4

Example 5 Construction of MaSp1 Genes

A DNA sequence encoding the Euprosthenops australis-derived protein 5Gly/Ala-CT_(nat) (SEQ ID NO: 9) was amplified by PCR with an Advantage GC2 kit (BD Biosciences, San Jose, Calif., USA), using a MaSp1 clone from the cDNA library of Example 1 as template. Restriction enzyme recognition sites BamHI and HindIII were introduced at the 5′- and 3′-ends, respectively, and a stop codon was introduced upstream of the HindIII site, by use of designed primers. The BamHI-5Gly/Ala-CT_(nat)-HindIII construct was then subcloned into a modified pET32 vector (Merck Biosciences, Darmstadt, Germany), prepared as described in Example 6(C) below.

Example 6 Construction of Chimeric MaSp1 Genes (A) REP Gene Fragments

DNA sequences coding for partial repetitive fragments (REP) denoted 3Gly/Ala and 4Gly/Ala were amplified by PCR with LA Taq (TaKaRa Bio; Saint Germain-en-laye, France) in the presence of betaine (Henke W et al, Betaine improves the PCR amplification of GC-rich DNA sequences. Nucleic Acids Res 25, 3957-3958 (1997)), using a partial cDNA clone encoding a repetitive region of Euprosthenops sp MaSp1 protein (Pouchkina-Stantcheva, NN & McQueen-Mason, SJ, Molecular studies of a novel dragline silk from a nursery web spider, Euprosthenops sp. (Pisauridae). Comp Biochem Physiol B Biochem Mol Biol 138, 371-376 (2004)) (GenBank entry CQ974358 or CQ816656) as template. Restriction enzyme recognition sites were introduced at the 5′- and 3′-ends, giving the following constructs: NcoI-3Gly/Ala-NheI and NcoI-4Gly/Ala-NheI to be joined with a CT fragment (see below); and a NcoI-4Gly/Ala-XhoI clone to be individually expressed, where a stop codon was inserted directly upstream of the XhoI site.

(B) CT Gene Fragments

A DNA sequence coding for the non-repetitive C-terminal domain from Euprosthenops sp (but with a high degree of similarity to MaSp1 from Nephila clavipes and Nephila senegalensis) was amplified by PCR using a genomic DNA clone encoding a C-terminal MaSp1 domain (Pouchkina-Stantcheva, NN & McQueen-Mason, SJ, Molecular studies of a novel dragline silk from a nursery web spider, Euprosthenops sp. (Pisauridae). Comp Biochem Physiol B Biochem Mol Biol 138, 371-376 (2004)). Restriction enzyme recognition sites were introduced at the 5′- and 3′-ends, giving NheI-2Gly/Ala-CT_(hyb)-XhoI, to be joined with the 3Gly/Ala and 4Gly/Ala partial REP clones (see above), and NcoI-2Gly/Ala-CT_(hyb)-XhoI, to be individually expressed.

(C) Construction of REP-CT hybrid MaSp1 Genes

The 3Gly/Ala and 4Gly/Ala REP clones were joined with the CT clones using the pCR®2.1-TOPO® vector (Invitrogen). Then, the resulting, fused 5Gly/Ala-CT_(hyb) and 6Gly/Ala-CT_(hyb) clones were excised with NcoI and XhoI, and subcloned into a modified pET32 vector (Novagen), where the original thrombin cleavage site was removed and a new thrombin site was introduced downstream of the enterokinase cleavage site.

Example 7 Expression of MaSp1 Fusion Proteins

The MaSp1 proteins coded for by the genes constructed in examples 5-6 were expressed as fusion proteins (of the type X-REP-CT) as follows, using a modified pET32vector: thioredoxin-tag/His-tag/S-tag/thrombin cleavage site/MaSp1 gene, encoding a thioredoxin/His₆/S-tag/thrombin cleavage site/MaSp1 protein, and an ampicillin resistance gene under control of the T7 promoter.

The different MaSp1 constructs in pET32 expression vectors were transformed into Escherichia coli BL21(DE3) cells (Merck Biosciences). The cells were grown at 30° C. in Luria-Bertani medium containing ampicillin to an OD₆₀₀ of 1.0-1.5, induced with IPTG and further incubated for 4 h at room temperature. The cells were harvested by centrifugation, and lysed by DNAseI and lysozyme in 20 mM Tris-HCl, pH 8.0, 20 mM imidazole, with 0.5 M NaCl, and further purified by His-tag affinity chromatography on Ni-NTA agarose (Qiagen, West Sussex, UK). Bound fusion proteins were eluted from the Ni-NTA column with 200 mM imidazole in 20 mM Tris-HCl, pH 8.0, with 0.5 M NaCl, and dialyzed against 20 mM Tris-HCl, pH 8.0. The resulting fusion proteins were >90% pure as judged by coomassie-stained SDS polyacrylamide gels and soluble in 20 mM Tris-HCl, pH 8.0. This process yielded approximately 40 mg/l culture of fusion protein, which was stable for weeks without significant precipitation.

In another experiment, the fusion proteins were expressed as His₆/thioredoxin/His₆/thrombin cleavage site/MaSp1 proteins from a plasmid containing the corresponding gene and a kanamycin resistance gene under control of the T7 promoter.

Example 8 Formation of Fibers from MaSp1 Proteins

Cleavage of the tags from the fusion proteins resulting from Example 7, was performed in 20 mM Tris-HCl, pH 8, with a thrombin:fusion protein ratio of 1:1000 (w/w), under very gentle rocking at room temperature. Thrombin cleavage was complete within 30-60 min, as judged by SDS-PAGE. The resulting MaSp1 proteins (FIG. 2B, SEQ ID NOS: 9-13) spontaneously polymerized into macroscopic fibers to varying extents, see Table 1. The fibers were initially formed at the water/air interface. The formation could be observed by the naked eye from about 1 hour of incubation (see FIG. 4A, 4B), and after about 5 hours occurred no further fiber growth. 6Gly/Ala-CT_(hyb) fibers were up to approximately 2 cm long, and 5Gly/Ala-CT_(nat) fibers were ≧10 cm long. Repeated experiments yielded 5Gly/Ala-CT_(nat) fibers that were 20 cm (see FIG. 4C), and even ≧2 m long. Fiber formation could be observed by the naked eye from about 10 min of incubation.

Fibers were isolated and washed with buffer and thereafter subjected to N-terminal amino acid sequence analysis, which showed only the sequence of the MaSp1 protein. This shows that the cleaved tags are absent in the fibers.

Example 9 Analysis of MaSp1 Protein Fibers

A. Tensile Strength Measurements

The tensile strength of the 6Gly/Ala-CT_(hyb) (SEQ ID NO: 13) and 5Gly/Ala-CT_(nat) (SEQ ID NO: 9) fibers of example 8 was determined as follows. In order to handle the shorter (1-2 cm) 6Gly/Ala-CT_(hyb) fibers for tensile strength measurements, they were incubated shortly in 15% glycerol in water before they were air-dried. The longer (10 cm) 5Gly/Ala-CT_(nat) fibers were either untreated, incubated shortly in 15% glycerol, or drawn by hand in 75% methanol before air-drying. Tensile strength of air-dried fibers was measured by pulling the fibers in a Zwick Material Tester at a rate of 10 mm/min. See Table 1.

The tensile strength of glycerol-treated air-dried 1-2 cm long fibers from 6Gly/Ala-CT_(hyb) (SEQ ID NO: 13) was about 2 MPa, and the strength of 10 cm fibers from 5Gly/Ala-CT_(nat) (SEQ ID NO: 9) was 4-5 MPa. Ten cm long 5Gly/Ala-CT_(nat) fibers drawn in the dehydrating solvent methanol before air-drying displayed a tensile strength of 2-3 MPa, which is slightly less than for glycerol-treated fibers of the same type. The highest tensile strength now measured was 10 MPa, which was found for an air-dried 10 cm long 5Gly/Ala-CT_(nat) fiber without further treatment.

The range of tensile strengths (2-10 MPa) is comparable to the lower values reported for regenerated spider silk fibers (2-320 MPa). The longest spontaneously formed fibers derive from the 5Gly/Ala-CT_(nat) construct, and such air-dried fibers also show the greatest tensile strength. Potentially, this could be due to its 12-15 residues long poly-Ala segments, relative the 8-14 residue Ala segments in 6Gly/Ala-CT_(hyb), which would give a greater proportion of crystalline β-sheet conformation in the former protein.

TABLE 1 Fiber forming capacity of MaSp1 proteins SEQ Fiber Fiber tensile ID Fiber forming length strength Protein NO capacity (cm) (MPa) 5Gly/Ala-CT_(nat) 9 ++++ ≧10  2-10 5Gly/Ala-CT_(nat) 9 +++++ ≧20 180-230 (Example 11) 4Gly/Ala 10 aggregates n.a. n.a. 2Gly/Ala-CT_(hyb) 11 aggregates n.a. n.a. 5Gly/Ala-CT_(hyb) 12 + ≦1 n.d. 6Gly/Ala-CT_(hyb) 13 +++ 1-2 ≦2 n.a. = not applicable n.d. = not determined B. Scanning Electron Microscopy

The microscopic architecture of the 6Gly/Ala-CT_(hyb) and 5Gly/Ala-CT_(nat) fibers was analyzed with scanning electron microscopy (SEM) (FIG. 5). Briefly, samples were applied on SEM-stubs and vacuum-coated with a 6 nm layer of gold and palladium. Specimens were observed and photographed in a LEO 1550 FEG SEM using an acceleration voltage of 10 kV.

This revealed diameters of 10-30 μm for single fibers, with individual fibers displaying rather homogenous diameters (FIG. 5a showing 6Gly/Ala-CT_(hyb), SEQ ID NO: 13). In addition to the macroscopic fibers, gel-like particles were found. After air-drying such particles of 6Gly/Ala-CT_(hyb) directly on a SEM-stub, fibers approximately 10-15 μm in diameter were seen (FIG. 5b, c). The diameter of macroscopic fibers of 5Gly/Ala-CT_(nat) (SEQ ID NO: 9), drawn in 75% methanol and air-dried, were 60-120 μm and they apparently contain several aligned fibers (FIG. 5d-f). Fiber twisted before air-drying (FIG. 5e), end of fiber (FIG. 5f).

C. Circular Dichroism Spectroscopy

Fibers consisting of 6Gly/Ala-CT_(hyb) protein (SEQ ID NO: 13) or 5Gly/Ala-CT_(nat) (SEQ ID NO: 9), prepared in Example 8, were washed in 20 mM phosphate buffer, pH 7, and suspended in 2% SDS in the same buffer. Circular Dichroism spectra from 250 to 190 nm were recorded at 22° C. in a 0.1 cm path length quartz cuvette, using a Jasco J-810 spectropolarimeter. The scan speed was 50 nm/min, response time 2 sec, acquisition interval 0.1 nm, and the band width 1 nm.

The spectrum shown in FIG. 6 is an accumulation of three scans of fibers of 6Gly/Ala-CT_(hyb) protein (SEQ ID NO: 13). It displays a minimum at 220 nm and a maximum at 195 nm, features that are characteristic of antiparallel β-sheet structures. Highly similar spectra were obtained for fibers of 5Gly/Ala-CT_(nat) (not shown). The spontaneously formed fibers thus exhibit similar morphology and structure as native and regenerated spider silk fibers.

Example 10 Biocompatibility of Recombinant Spider Silk

Since it is desirable to use spider silk fibers in biomedical applications, the biocompatibility of the fibers has been evaluated by an investigation of effects of recombinantly produced silk using two different cell types.

The MaSp1 protein 5Gly/Ala-CT_(nat) (SEQ ID NO: 9) was expressed in bacteria as described in examples 7-8. Puri-fied protein was used to produce artificial silk fibers with lengths of >10 cm, and even >20-200 cm, and diameters of around 100 μm.

A. Embryonal Mouse Mast Cells

Embryonal (day 12.5) mouse mast cells (in vitro proliferated for eight weeks using IL-8 and mast stem cell factor) were seeded at two different cell densities, the higher density being about four times the lower density. These cells do not adhere to the plastic surface, but grow in suspension. Pieces of the silk fiber, each about 0.5 cm long, were added to the wells. Mast cells were incubated for three days, with or without the presence of silk fiber, and thereafter living and dead cells were counted after staining with Trypan blue (FIG. 7). The bars show the mean values with standard error mean, n=2, each sample is counted in triplicate.

The mast cells are not affected by the presence of the silk fibers. After three days of growth, there are no significant differences in cell death or proliferation compared to the negative controls grown without silk fibers.

B. Human Embryonic Kidney (HEK) 293 Cells

Pieces of the silk fiber, about 0.5 cm long, were adsorbed to the bottom of 6-well microtiter plates by letting them dry from a small volume of buffer. The fibers do not detach when cell growth media is added. Human embryonic kidney (HEK) 293 cells were then plated at different cell densities and allowed to grow for a total of six days. The HEK-293 cells adhere and grow attached to the plastic cell surface. The ability of the HEK-293 cells to grow in the proximity of the fibers, and the physical attachment of the cells to the fibers was studied.

The HEK-293 cells attached and proliferated normally in the wells containing silk fibers (as observed under the light microscope). The cells grew very closely along the fiber edges, and apparently even grew under a partly detached fiber (FIG. 8). After seven days, the fibers were carefully detached from the plastic surface, and it was clearly seen that groups of cells were physically attached to the fibers. The fiber covers the upper right half of the figure. HEK293-cells are seen attached to the edge of the fiber, and also grow under the fiber.

The two different cell types (mast cells, HEK-293) studied were not affected by the presence of recombinant silk fibers, even at comparatively high amounts of silk. This indicates that the tested artificial silk fibers resemble wild type dragline silk of Euprosthenops australis, in being non-toxic and biocompatible. The artificial silk fibers thus appear suitable for biomedical applications.

Example 11 Mechanical Properties and Structure of MaSp1 Protein Fibers

The mechanical properties of fibers from 5Gly/Ala-CT_(nat) (SEQ ID NO: 9) were examined using tensile tests performed to yield stress-strain curves (FIG. 9). The tensile properties were characterized using a Zwick Roell Z2.5 material tester (Zwick, Ulm, Germany). The tests were performed in air at ambient conditions (20° C. and 52% relative humidity) using a loading speed of 10 mm/min. Fiber pieces were transferred directly from buffer, mounted and subjected to two stretching-relaxation cycles. In order to generate a homogenous silk thread suitable for tensile testing, the fibers were elongated using stretch-relaxation cycles. First, the fibers were elongated by pulling up to a force of 0.1 N. After relaxation, they were further drawn until a force of 0.25 N was applied.

This treatment generated elongated homogenous fibers with a diameter of approximately 80 μm as determined by height measurements using a Mitutoyo IDC-112B instrument (Mitutoyo Corp, Tokyo, Japan) and confirmed by scanning electron microscopy (SEM) as follows. Before and after stretch-relaxation cycles, fiber pieces were applied on SEM stubs and air-dried overnight. The samples were vacuum-coated with a 6 nm layer of gold and palladium. Specimens were observed and photographed with a LEO 1550 FEG microscope (Carl Zeiss, Oberkochen, Germany) using an acceleration voltage of 10 kV.

The drawn fibers were cut into pieces, the ends of which were fixed between cardboard paper with glue (Loctite 420, Loctite, Goteborg, Sweden). Fiber samples were then fixed in the grips of the material tester and stretched until they broke. Stress-strain curves were constructed using the initial cross-sectional area of the pre-drawn fibre, assuming a circular cross-section. The stress values are normalized to the initial cross-sectional area of the fiber. The strain corresponds to dL/L₀ where L₀ is the initial length of the fiber and dL is the change in fiber length. In FIG. 9, the stress-strain curves for three different samples of double drawn fibers of 5Gly/Ala-CT_(nat) (SEQ ID NO: 9) are shown, and their tensile strength measured approximately 0.2 GPa.

The microscopic architecture of the fibers was analyzed by SEM (FIG. 10). The spontaneously formed fibers have a homogenous flattened appearance and a width of up to several hundred micrometers, while the height measures some ten micrometers (FIG. 10 a,b).

After the fibers had been subjected to stretch-relaxation cycles, their cross section adopted a more rounded shape with a compact substructure of tightly aligned fibrils (FIG. 10 c-f). The appearance of cut or fractured surfaces (FIG. 10 e,f) further attest to the compactness of the produced fiber.

In conclusion, the spontaneously formed fibers show similar morphology and mechanical properties as native or regenerated spider silk fibers, even without spinning.

Example 12 Spidroin Protein Variants

Strong intermolecular interactions are thought to contribute to the impressive tensile strength of spider silk. Therefore, variants of miniature spidroins that allow intermolecular covalent cross-linking in the fibers have been produced. Two different mutant spidroin proteins have been constructed by site-directed mutagenesis to introduce two cysteine residues in the first (SEQ ID NO: 14, positions 36 and 37) and the fourth (SEQ ID NO: 15, positions 128 and 129) alanine block, respectively. These variants have been expressed and isolated using the same protocol as described in Examples 7-8 for the genes constructed in Examples 5-6.

These variants (SEQ ID NOS: 14-15) form fibers in the same manner as 5Gly/Ala-CT_(nat) (SEQ ID NO: 9).

In order to elucidate the importance of dimerization of the C-terminal domain, a variant where the cysteine residue in the C-terminal domain is exchanged for a serine residue has been constructed (SEQ ID NO: 16, position 222). However, this variant (SEQ ID NO: 16) forms fibers in the same manner as 5Gly/Ala-CT_(nat) (SEQ ID NO: 9).

Example 13 Removal of LPS and Other Pyrogens from Expressed Spidroin Proteins

E. coli cells expressing the desired spidroin fusion protein are washed with the following buffers:

A: 100 mM Tris, pH 8,

B: 5 mM CaCl₂, 100 mM Tris, pH 8,

C: 10 mM EDTA, 100 mM Tris, pH 8,

D: 100 mM Tris, pH 8, and

E: 100 mM Tris, pH 8.

Thereafter, the cells are lysed in 20 mM Tris, pH 8 supplemented with lysozyme and DNaseI. The protein sample is then loaded on a Ni-sepharose matrix and washed with 20 mM Tris, 10-100 mM imidazole, pH 8 before elution with 20 mM Tris, 100-300 mM imidazole, pH 8. Relevant fractions are pooled and dialyzed against 20 mM Tris, pH 8 over night. The protein sample is then supplemented with 100 μM CaCl₂ and finally passed through an EndoTrap Blue column, previously equilibrated with 20 mM Tris, 100 μM CaCl₂, pH 8. In this way, protein samples with a pyrogen content of 1 EU/mg protein can be obtained, as judged by IPT and a LAL kinetic assay.

The fusion protein is then proteolytically cleaved with thrombin using a 1:1000 (w/w) thrombin:fusion protein ratio, which induces fiber formation (as described above). The fibers are washed 3 times in 20 mM Tris, pH 8 and finally 3 times in water. This gives fibers with a pyrogen content of 0.25 EU/mg fiber.

The structural characteristics of the fibers are unaffected after autoclaving at 125° C. and 1.5 bar for 10 min, which enables efficient sterilization of the material. The fibers are chemically stable and can not be solubilized in either of 8 M urea, 6 M GuaHCl, or neat HAc. However, the fibers can be solubilized in neat HFIP or formic acid. 

1. An isolated major ampullate spidroin protein comprising the amino acid sequence as set forth in one of SEQ ID NO: 17, SEQ ID NO: 18, SEQ ID NO: 19 or SEQ ID NO:
 20. 2. An isolated fusion protein comprising the protein according to claim 1 and a protein fragment comprising a fusion partner and a cleavage agent recognition site; wherein the protein is coupled via said cleavage agent recognition site to said fusion partner.
 3. A polymer of the major ampullate spidroin protein according to claim
 1. 4. A composition comprising the isolated protein according to claim 1, wherein the content of lipopolysaccharide (LPS) and other pyrogens is 1 EU/mg of isolated protein or lower.
 5. A method of producing a soluble fusion protein as defined in claim 2, comprising the steps of: (i) expressing a nucleic acid molecule which encodes the fusion protein according to claim 2 in a suitable host; and (ii) isolating the soluble fusion protein obtained in step (i), optionally involving removal of lipopolysaccharide (LPS) and other pyrogens.
 6. A method of producing a polymer of a major ampullate spidroin protein, comprising the steps of: (i) providing a solution of the fusion protein according to claim 2 in a liquid medium, (ii) adding to said liquid medium a suitable cleaving agent for achieving cleavage of the fusion protein at the cleavage agent recognition site, and thereby obtaining the major ampullate spidroin protein; (iii) allowing the major ampullate spidroin protein obtained in step (ii) to polymerize in the liquid medium; and optionally (iv) isolating the polymer obtained in step (iii) from said liquid medium, optionally involving removal of lipopolysaccharide (LPS) and other pyrogens.
 7. The method according to claim 6 of producing a polymer of a major ampullate spidroin protein, wherein step (i) comprises: (a) expressing a nucleic acid molecule which encodes said fusion protein in a suitable host; (b) isolating the soluble fusion protein obtained in step (a), optionally involving removal of lipopolysaccharide (LPS) and other pyrogens; and (c) providing a solution of said soluble fusion protein obtained in step (b) in a liquid medium.
 8. The method according to claim 7, wherein said step (iii) of allowing the major ampullate spidroin protein obtained in step (ii) to polymerize in the liquid medium, further comprises providing an interface between said liquid medium and another phase selected from the group consisting of a gas phase, a liquid phase and a solid phase, wherein said polymerizing initiates at said interface or in a region surrounding said interface.
 9. The method according to claim 7, wherein said liquid medium is an aqueous medium and said other phase is selected from the group consisting of air and water-immiscible organic solvents. 